1. Field of the Invention
This invention relates to an optical amplifier and an optical amplification method, and more specifically to 1.58 .mu.m band optical fiber amplifiers for amplifying signal light in a wavelength band of 1.570 to 1.600 .mu.m or 1.565 to 1.600 .mu.m and broad-band optical fiber amplifiers using the 1.58 .mu.m band optical fiber amplifiers. Also, this invention relates to amplification methods using such optical fiber amplifiers.
2. Description of the Related Art
An Er-doped optical fiber amplifier (EDFA) has advantageous characteristics that it is possible to effect optical amplification in 1.55 .mu.m where transmission loss of communication optical fiber is minimum, it has no dependence on polarization, it can be connected with a communication optical fiber with a low loss, and the like. Therefore, the Er-doped optical fiber amplifier is being developed as an optical amplifier for 1.55 .mu.m band optical communication systems, and widely applied in, for example, a trunk transmission system for transmitting digital signals, an optical CATV transmission system for transmitting analog signals, and the like.
Recently, in the 1.55 .mu.m band optical communication system, with the aim of even further increasing the communication capacity, transmission of wavelength division multiple (WDM) signals comprising a plurality of optical signals of different signal wavelengths is being attempted. For this purpose, the optical fiber amplifier is required to have a wide amplification bandwidth, and characteristics for amplifying individual signal lights differing in wavelength in WDM signals without gain deviation.
To overcome such requirements, the following conventional methods have been proposed:
(1) A method in which an Er-doped silica based fiber that is codoped with Al is used as an amplification medium; PA1 (2) A method in which a gain equalizer is added behind the optical fiber amplifier; PA1 (3) A method in which an Er-doped fluoride based fiber is used as an amplification medium; and PA1 (4) A method in which two Er-doped optical fibers differing in gain spectrum are cascade connected to each other. PA1 an erbium doped fiber having a core part and a clad part, at least one of the core part and the clad part being doped with erbium, PA1 an excitation light source for exciting the optical fiber, PA1 optical means for inputting excitation light from the excitation light source and signal light to the Er-doped fiber, and PA1 an optical isolator, PA1 wherein the erbium doped fiber is a 1.58 .mu.m band optical fiber having an equivalent fiber length as a product of a fiber length (m) and an erbium doping concentration (ppm by weight), the equivalent fiber length providing a signal gain, obtained at a wavelength of the excitation light source used for excitation of the erbium doped fiber, of more than a predetermined practical reference value. PA1 a demultiplexer for dividing signal light into plurality of wavelength regions; PA1 a plurality of optical amplification units for amplifying individual signal light outputted from the demultiplexer; and PA1 a multiplexer for combining individual signal light outputted from the plurality of optical amplification units: PA1 wherein at least one of the optical amplification unit may comprise an erbium doped fiber having a core part and a clad part, at least one of which is doped with erbium, an excitation light source for exciting the optical fiber, optical means for inputting excitation light from the excitation light source and signal light to the Er-doped fiber, and an optical isolator and wherein the erbium doped fiber is a 1.58 .mu.m band optical fiber having an equivalent fiber length as a product of a fiber length (m) and an erbium doping concentration (ppm by weight), the length providing a signal gain obtained at a wavelength of the excitation light source used for excitation of the erbium doped fiber of no smaller than a predetermined practical reference value. PA1 wherein the erbium doped fiber has an equivalent fiber length as a product of a fiber length (m) and an erbium doping concentration (ppm by weight), the length providing a signal gain obtained at a wavelength of the excitation light source used for excitation of the erbium doped fiber of more than a predetermined practical reference value.
In relation to the method (1) above, it is reported that the gain deviation of 1.54-1.56 .mu.m WDM signals can be reduced to less than 0.2 dB (S. Yoshida, et al., "Gain flattened FDFA with high Al concentration for multistage repeated WDM transmission systems", Electron. Lett., vol. 31, pp. 1765-1767, 1995). On the methods (2) and (3) above, thebackwarde reports that the gain deviation of 1.532-1.56 .mu.m WDM signals can be reduced to less than 1.5 dB (R. Kashyap, et al., "Wideband gain flattened erbium fiber amplifier using a photosensitive fiber blazed grating", Electron. Lett., vol. 29, pp. 154-156, 1993 and M. Yamada, et al., "Fluoride-based erbium-doped fiber amplifier with inherently flat gain spectrum", IEEE Photonics Technol. Lett., vol. 8, pp. 882-884, 1996). Further, as for the method (4) above, there have been reported that an Al,Er-codoped silica-based fiber and a P,Er-codoped silica-based fiber in a cascade configuration can achieve a gain deviation in 1.54-1.56 .mu.m WDM signal of less than 1.3 dB (T. Kashiwada et al., iGain flattening for fiber amplifiersi, OFC i95, paper TuP1, 1995), and that use of an Al,Er-codoped silica-based fiber and an Er-doped fluoride-based fiber in a cascade configuration can achieve a gain deviation in 1.53-1.56 .mu.m WDM signal of less than 1.1 dB (M. Yamada et al., iGain flattening for fiber amplifiersi, IEEE Photonics Technol., Vol. 8, pp. 620-622, 1996).
Presently, there have been reported WDM transmission experiments at 1 Tbit/s, 2.6 Tbit/s, and so on using optical fiber amplifiers according to the methods (2), (3) and (4) above. However, even with the this method, the gain-flattened amplification band is limited to 1530 nm to 1560 nm as shown in FIG. 1 (Yamada, et al., Proceedings for the Electronic Information Communication Society, LQE 96-27, pp. 43-48) so that in order to even further expand the communication bandwidth used in WDM transmission, improvements in the band characteristics including the broadening of band and the flattening of gain, of optical fiber amplifiers are important.
Furthermore, there has been a report that Er-doped silica based fibers have an amplification band of 1.57 to 1.61 .mu.m (1.58 .mu.m band), in addition to the amplification band of 1.53 to 1.56 .mu.m (1.55 .mu.m band) (J. F. Massicott, et al., "High gain, broadband, 1.6 .mu.m Er.sup.3+ -doped silica fibre amplifier", Electron. Lett., vol. 26, No. 20, pp. 1645-1646, 1990 and J. F. Massicott et al., iLow noise operation of Er.sup.3+ doped silica fibre amplifier around 1.6 .mu.mi, Electron. Lett., Vol. 28, No. 20, pp. 1924-1925, 1992). FIG. 2 shows an amplification band of 1.58 .mu.m band shown in this report (J. F. Massicott, et al., "High gain, broadband, 1.6 .mu.m Er.sup.3+ -doped silica fiber amplifier", Electron. Lett., vol. 26, pp. 1645-1646, 1990). In FIG. 2, the solid line, broken line and dashed line indicate signal gain curves where the fiber length L=200 m, 175 m, or 150 m, respectively.
However, the conventional 1.58 .mu.m band optical fiber amplifiers have the following problems.
1) A high power 1.55 .mu.m band pump light source is required, which is expensive, unlike those of a 0.98 .mu.m band (Er .sup.4 I.sub.11/2 level excitation of the Er-doped silica based fiber), 0.97 .mu.m band (Er .sup.4 I.sub.11/2 level excitation of the Er-doped fluoride based fiber), or a 1.48 .mu.m band (Er .sup.4 I.sub.11/2 level excitation of individual fibers) used as an excitation light source for the conventional Er-doped optical fiber amplifier.
2) A definitive designing method for the fiber length and other parameters of the Er-doped optical fiber for achieving the 1.58 .mu.m band amplification has yet to be clarified.
3) The gains at respective signal wavelengths of WDM signals in the 1.58 .mu.m band cannot be flattened (the gain deviation cannot be reduced).